1. Field of the Invention
This invention relates generally to an optical scanner that causes a beam of light to scan across a surface at uniform velocity, and more particularly to a dust countermeasure in an optical scanner having a structure that turns back the optical path of a light beam deflected by an optical deflector such as a rotating polygon mirror.
2. Description of the Related Art
Hitherto, optical scanners to perform optical scan with a light beam deflected by a deflector using a polygon mirror (rotating polygon mirror) are well known. This type of optical scanner is employed in recorders or readers that use a laser beam, etc. The general construction consists of a light source for emitting a light beam, a polygon mirror for deflecting the light beam emitted from the light source, and a scanning optics system (imaging optics system) employing an fxcex8 lens, a correction lens, etc. The fxcex8 lens is used for focusing the deflected light beam on a predetermined surface and causing the focused light beam to scan on the surface at uniform velocity, and the correction lens is used to correct the positional error (pitch unevenness) in the scanning line due to the surface tilt of the deflector.
If the fxcex8 lens, etc., of the scanning optics system are disposed simply in a row on the optical axis of the light beam deflected by the polygon mirror, the size of the scanning optics system will increase in this optical-axis direction.
Hence, for example, as shown in FIG. 4, the required optical path length has been ensured and the scanning optics system reduced in size, by (1) inserting two reflecting mirrors 132, 133 into the scanning optics system; (2) obliquely reflecting a light beam L1, deflected by a polygon mirror 120 and passed through an fxcex8 lens 131, by the first reflecting mirror 132; and (3) further reflecting the reflected light beam by the second reflecting mirror 133 so that the optical path of the reflected light beam L2 crosses the optical path of the light beam L1 incident on the first reflecting mirror 132 (e.g., Japanese Unexamined Patent Publication No. 3(1991)-220517).
However, as shown in FIG. 4, one (in this example, the first reflecting mirror 132) of the two reflecting mirrors 132, 133 must be directed toward a ceiling side in order to reflect the incident light beam L1 and cross the reflected light beam L2 with the incident light beam L1, and dust is liable to accumulate on the surface of reflection of the reflecting mirror facing the ceiling side. As a consequence, the problem of image unevenness, signal-to-noise (S/N) ratio degradation, etc., will arise in readers, etc., which use an optical scanner having such structure.
The present invention has been made in view of the aforementioned problem. Accordingly, it is the primary object of the present invention to provide an optical scanner having structure which turns back the optical path of a light beam, while solving the problem of dust adhesion.
In the optical scanner of the present invention, a reflecting prism, which serves as a means of turning back a light beam emitted from a deflector, is inserted between a deflector and a scan surface. The reflecting prism reflects the light beam, reflected at the reflector, in the opposite direction from the incident direction of the light beam, and the reflecting mirror reflects the light beam reflected by the prism toward the scan surface. With this arrangement, the required optical path length is ensured and the problem of dust adhering to the reflection surface facing a ceiling side is solved.
More particularly, the optical scanner of the present invention comprises: a light source for emitting a light beam; a deflector for deflecting the light beam in a predetermined direction; and a scanningoptics system for causing the deflected light beam to scan on a predetermined surface at a uniform velocity. The scanning optics system includes a reflecting prism having at least two reflection surfaces which reflect the light beam, incident from the deflector, in the opposite direction from the incident direction of the light beam.
The words xe2x80x9copposite directionxe2x80x9d mean that the optical axis of the light beam emitted from the reflecting prism has at least the component of the opposite direction from the optical axis of the light beam incident on the reflecting prism and do not always mean that the optical axis of the emitted light beam is in the exact opposite direction from the optical axis of the incident light beam.
The xe2x80x9creflecting prismxe2x80x9d may be of any type, as long as it has at least two reflection surfaces so that an incident light beam is totally reflected at the reflection surfaces and emitted in the opposite direction from the direction of the incident light beam. For example, the reflecting prism may employ a triangular prism, a trapezoidal prism, etc., which have an incidence surface on which a deflected light beam is incident, a first reflection surface to reflect the light beam passed through the incidence surface, and a second reflection surface to reflect the light beam reflected by the first reflection surface toward the incidence surface.
The optical scanner of the present invention may further include a dust-proof cover for protecting at least one of at least two reflection surfaces which is disposed on a ceiling side, the dust-proof cover being disposed via the aforementioned at least one reflection surface and a gap so that the aforementioned at least one reflection surface is covered.
In addition, it is preferable for the optical path of the light beam, which is incident on the reflecting prism and reflected within the reflecting prism, to be set so that no resonance occurs within the reflecting prism.
The expression xe2x80x9cset so that no resonance occursxe2x80x9d means that the relationship between the configuration of the reflecting prism and the position of incidence and angle of incidence of a light beam incident on the reflecting prism is set so that no resonance occurs within the reflecting prism.
Furthermore, in the optical scanner of the present invention it is preferable that the diameter of the light beam at the incidence surface of the reflecting prism (diameter of a point at which the light intensity I of the beam center becomes 1/e2 where e is the base of a natural logarithm) ranges from 2 mm to 10 mm.
Moreover, in the optical scanner of the present invention, the scanning optics system may further include a reflecting mirror which reflects the optical beam, reflected (turned back) by the reflecting prism, toward the scan surface.
According to the optical scanner of the present invention, the reflecting prism is used for reflecting the light beam deflected by the deflector, in the opposite direction from the incident direction of the light beam. Therefore, there is no possibility that dust will accumulate on the reflection surface facing a ceiling side. As a result, there is no possibility that the problem of image unevenness, S/N ratio degradation, etc., will arise in readers, etc.
Also, since the reflection surface of the reflecting prism positioned on the ceiling side is protected with a dust-proof cover, the problem of dust accumulating on the outside of this reflection surface and having an adverse influence on total reflection will not occur.
In addition, if the relationship between the configuration of the reflecting prism and the position of incidence and angle of incidence of a light beam incident on the reflecting prism is set so that no resonance occurs within the reflecting prism, the occurrence of noise due to light beam resonance within the reflecting prism can be prevented.
Furthermore, even if microscopic dust adheres to the incidence surface of the reflecting prism, in the case where the diameter of the light beam at the incidence surface of the reflecting prism ranges between 2 mm and 10 mm, the light beam would not be influenced by this dust, and a compact, inexpensive optical scanner can be constructed.